This project studies the effect of behavioral and environmental variables on responses of thalamic projection neurons in the medullary dorsal horn (trigeminal nucleus caudalis) to noxious and innocuous thermal stimuli. Rhesus monkeys are trained to detect the termination of innocuous heat stimuli and the onset of noxious heat stimuli. In a second task, these monkeys detect the onset of a light stimulus. Other monkeys are being trained to discriminate which of two simultaneiously presented heat pulses is hotter and which of two simultaneously presented lights is brighter. Tgrigeminothalamic neurons code thermal discriminative information used by the monkey within the behavioral tasks. Some neurons that respond to passive mechanical and thermal stimulation also respond to other stimuli the monkey uses for successful completion of the task. We observe several patterns of task-related activity, and some neurons showing each pattern project to the thalamus. These task-related responses may modulate sensory activity and thereby influence the perception of and response to oral-facial pain.